1. Field of the Invention
The present invention relates generally to fluid flow control. The present invention relates specifically to a practical pneumatic oscillator for generating large amplitude pressure and velocity oscillations suitable to prevent stalling on airfoil and diffuser surfaces.
2. Discussion of the Related Art
Active air flow control is an area of intense research in the aerospace industry. In particular, air flow control using pulsed air injection through slots and holes on various surfaces of the aircraft and aircraft engines is known to improve performance when operating near stalled conditions. Air flow control with pulsed air injection has been demonstrated under laboratory conditions to control forebody flow vortices on aircraft and missiles at high angles of attack, delay the stall of wings on aircraft, enhance the lift characteristics of helicopter blades, suppress the stall in engine compressor inlets, and enhance the performance of vanes in axial flow compressors.
The aerospace industry is attempting to transition laboratory results of flow control experiments into prototype applications. The principal difficulty in most cases has been to produce large amplitude velocity oscillations at the air-to-surface interface with equipment that is practical to be carried in the aircraft. In the laboratory one typically uses large loudspeakers and acoustic drivers with massive power amplifiers, or a large rotating valve with a drive mechanism to produce the pressure oscillations. One problem with the speaker approach is that the equipment needed is too large. Secondly, the speaker techniques are limited by the push-pull nature of the loud speaker diaphragms to low frequency bandwidths which produce only small amplitude oscillations superposed on a steady jet of air. Similarly, known techniques utilizing a single valve in a single pressurized line produce only small amplitude oscillations superposed on a steady jet of air. When the frequency of the single valve opening goes up, the amplitude of the pressure changes goes down. What is needed is a device that is sufficiently compact and simple enough to be practical for implementation on aircraft while producing pressure oscillations near the maximum possible amplitude.
The present invention solves the above problems by providing both a device and a process for using the device. In broad outline, the device comprises a pair of activatable valves connected in first and second parallel lines downstream from a pressurized air supply. xe2x80x9cParallelxe2x80x9d will be understood to mean that the lines are interconnected and could share a parallel flow, and not that the lines must be physically parallel. The downstream side of the first valve is connected via a suitable air conduit, or actuator line, to an actuator outlet which provides pulsed air, to the controlled surface. The controlled surface may be, e.g., an airfoil surface or a diffuser surface such as a vane or compressor as may be found on turbine engines. This conduit between the first valve and the actuator may be referred to as the actuator line. The actuator line typically leads to the controlled surface through a thin slot, sometimes herein called the actuator outlet, of smaller dimensions than the actuator line. Thus, by way of explanation and not limitation, it is believed that as pressure rises in the actuator line, air velocity will increase through the actuator outlet at the controlled surface. The downstream side of the second valve can be either open to the atmosphere or connected to a vacuum line or other source, or area, of lower pressure. A controller is used to set the activation and frequency of oscillation of the two valves and the phase difference between the two valves. In some embodiments the duty cycles of the valves may also be controlled.
Pressurized air is suitably provided by an onboard source such as may be commonly found in the compressor stages of jet engines or otherwise provided, such as by a dedicated onboard compressor. The supply line for the pressurized air is then preferably fed to a pressure regulator to supply an output stream of known pressure. The regulator may provide a fixed value pressure or may be adjustable, such as through control signal input from the controller function. The proper operation of the device then produces the needed amplitude of pneumatic oscillations at the actuator.
The process, or operational, component of the invention uses offset timing of the second valve, from activation of the first valve, to create an air-ejector effect through the second line thereby decreasing pressure in the actuator line. Typically the opening and closing of the second valve will lag the opening and closing of the first valve by about a quarter cycle, or about ninety degrees. The large amplitude oscillations occur because the second valve acts like an air ejector when it is open, which lowers the pressures in each parallel line, and concurrently increases velocity through the second line by the Bernoulli principle, and particularly for purposes of the present invention, lowers the pressure in the first parallel line. The combined high and low pressure phases of the two line system, due to the offset opening and closing of the valves, produce pressure oscillations in the first, or actuator line, of an amplitude and frequency sufficient for creating air velocity pulses at the actuator outlet to control air flow at the airfoil or diffuser surfaces.